Whenever possible, make adjustments with the
in place removing it alters the partial
inside the carburettor.
On cars where the carburettor is not very accessible, you may have to remove the air cleaner to reach some parts, then carry out further fine adjustments to obtain correct engine running later on.
Checking the mixture
Use the piston lifting pin at the side of the carburettor to check the fuel-air mixture.
With the engine running at working temperature, hook your finger under the pin and raise the piston about 1/32in. (1 mm).
If, while the piston is raised, the engine speed increases briefly then returns to normal, the mixture is correct. If it rises and stays high, the mixture is too rich. If it falls and the engine tends to
, the mixture is too weak.
To correct the mixture, move the jet adjuster nut one hexagon flat at a time.
Screwing it up - anti-clockwise as seen from above - makes the mixture weaker.
Screwing it down -
makes the mixture richer.
Each time you move the adjuster, wait for about ten seconds, then check again with the lifting pin to see whether the mixture is now right.
Take care that the engine temperature does not rise above normal - which it will if you take too long.
Such a rise will result in a false, overweak mixture setting which will show when the temperature returns to normal.
Before and after adjustment, check that the jet needle is central in the jet.
Stop the engine, use the
to lift the piston to the top of its travel and let it drop.
It should fall smoothly with a sharp click. If it does not, the jet is out of line and you must centralise it.
An off-centre jet may also
or bend the tapered needle. Do not attempt to clean up or straighten a badly scored or bent needle. It must be replaced (See
Checking and cleaning an SU carburettor
Centralising the jet
Remove the air cleaner and the dashpot screw-in top and damper. Use a screwdriver to raise the piston.
Turn the jet adjuster nut up as far as it will go, or until the jet is level with the bridge inside the carburettor.
Slacken the large locking nut above the adjuster nut on the jet where it enters the carburettor body.
Now use a pencil or soft metal rod, such as a stick of solder, to push the piston right down. Hold it down and tighten the locking nut.
Check that the piston drops with a click - if it does not, repeat the centralising process.
Screw the jet adjuster nut down two full turns, which should bring it near enough to the correct setting for the engine to be able to run.
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resembles the simpler variable-jet type (See
How variable-jet carburettors work
) in having a
- a constricted neck - through which air flows on its way to the